Highly Selective Hydrogenation of R-(+)-Limonene to (+)-p-1-Menthene in Batch and Continuous Flow Reactors

In our study, heterogeneous catalysts based on different active metal nanoparticles (Pt, Pd, and Ru) and supports (carbon, silica, and alumina) were systematically tested in hydrogenation of limonene under mild reaction conditions (room temperature, 3 bar H-2). The heterogeneous catalyst Pt/C was found highly active and selective for the reduction of limonene into the partial hydrogenation product (+)-p-1-menthene. Pt/C and Pt/Al2O3 are the most active systems among the series of commercially available catalysts that have been investigated. The catalytic activity and stability of Pt/C remained high throughout the recycling tests under batch conditions and thus allowed for the implementation of this catalytic system into continuous flow operations. The selective hydrogenation of the terminal over the internal C=C bond of limonene was rationalized by detailed kinetic studies which revealed an 8-fold difference in reaction rate between the two reactions.